JP-A-2001-295960 discloses a valve unit, which has a chamber for absorbing variation in pressure therein. The chamber needs a volume sufficient for absorbing variation in pressure. In this structure of the valve unit, rigidity of the valve housing may become low due to defining the chamber therein. The valve unit includes a solenoid actuator. When the solenoid actuator is supplied with electricity so that a valve member collides with a valve seat in the valve unit, the collision causes vibration in the valve housing. The vibration further causes radiation of large operation noise in the valve housing being low in rigidity. The vibration in the valve housing is transmitted to a fixed member mounted with the valve unit, and causes vibration in the fixed member. As a result, the fixed member causes large operation noise due to transmission of the vibration.